What is the Function of a Joint?

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Personal/Fitness Training Blog

Joints are the points of the body where two bones meet. There is often movement between them but sometimes there is not. A joint has two main functions: to allow mobility of the skeletal system and to provide a protective enclosure for vital organs.

How are joints classified?

Joints are divided into three categories:

  1. Immovable or Fibrous Joints

These joints are called ‘fixed’ or ‘immovable’ joints because they do not move. They have no joint cavity and are connected via fibrous connective tissue. The skull bones are connected by fibrous joints.

  1. Cartilaginous Joints

These joints also have no joint cavity and the bones are connected tightly to each other with cartilage. These joints only allow a small amount of movement which means that they are called ‘partly’ or ‘slightly movable’ joints. The vertebrae are examples of cartilaginous joints.

  1. Synovial Joints

Most of the joints in the body are synovial joints. These joints are ‘freely movable’ and are characterised by being surrounded by an articular capsule which contains the synovial fluid. This fluid:

  • Lubricates the joints,
  • Supplies nutrients to the cartilage, and
  • Contains cells that remove microbes and debris within the joint cavity.

Because of the larger range of movements of these joints there is an increased risk of injury, e.g. dislocations. Synovial joints are located predominantly in limbs.

Many synovial joints also have ligaments either inside or outside the capsule.

The range of movement provided by these joints is determined by:

  • The closeness of the bones at the point of contact: Closer bones make stronger joints but movements are more restricted. The looser the fit the greater the range of movement. However, looser joints are more prone to dislocation.
  • The flexibility of the connective tissue and the position of the ligaments, muscles and tendons.

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What are the types of synovial joints?

There are six types of synovial joints. These are:


These joints allow a singular plane of motion, in other words flexion and extension. Hinge joints can be found in the elbow and knee. The simple push-up will demonstrate how the hinge joint works. Here’s how to do this exercise:

  • Place your hands on the group about shoulder-width apart.
  • Balance on your toes with your feet slightly apart.
  • Tighten your glutes as you lower your body towards the ground until your chest and hips almost touch the ground.
  • Keep your elbows close to your body.
  • Exhale and push yourself off the group. Keep you head aligned with your spine and hips.

Ball and Socket

Ball and Socket Joints permit a free range of movement, in other words flexion, extension, abduction, adduction and circumduction. These joints can be found in the shoulders and hips. Want to know how this type of joint works? Doing the side-lying leg circles will show you. To do this exercise:

  • Put your legs at a 45-degree angle.
  • Bend your supporting leg and extend your top leg.
  • Use you core muscles to stabilise your pelvis.
  • Perform five forward and five backward leg circles on each leg.


This type of joint can be found in the neck. These allow for uni-axil rotation, in other words moving from side to side.


Plane joints allow short gliding or slipping motions. This is because the surfaces of the bones are flat. An example of where in the body a plan joint can be found is the vertebrae.


These joints allow for movement in two planes of motion, in other words flexion and extension, adduction or abduction. You’ll find condyloid joints in the knuckles.


Saddle joints allow for movement in two directions, as can be found in the thumbs.

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As a personal trainer, you need to have an in-depth knowledge of how joints function as these are integral to exercise. Want to learn more about becoming a personal trainer? Check out Trifocus Fitness Academy’s Personal Training Course! To learn about our other online fitness courses, visit our website.

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